Crossword Genius

Are they all called Jack? (4)

Ross

I believe the answer is:

tars

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'are they' is the definition.
The definition suggests a plural noun which matches the answer.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for tars that I've seen is " Sailors".)

I've seen this clue in the Evening Standard.
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